1. Technical Field
An embodiment of the present invention relates to a frequency measuring device etc. provided with a plurality of sets of a resonator having an adsorption film and an oscillation circuit connected to the resonator.
2. Related Art
A phenomenon occurs in which, when a substance contained in a surrounding medium is attached to the surface of a quartz crystal resonator under a resonant condition, the resonance frequency changes according to the attached substance. A technique using this phenomenon is called QCM (quartz crystal microbalance) and used as a sensor detecting the presence and the amount of molecules contained in the surrounding medium. As the application of QCM, there is an odor sensor having a resonator with a surface on which an adsorption film which selectively adsorbs a particular molecule is formed. Moreover, the application of QCM to a biosensor, a gas sensor, etc. using DNA hybridization has also been studied. Hereinafter, descriptions will be given by taking up mainly the odor sensor as an example.
In general, an AT cut quartz crystal resonator is used in a QCM device. AT cut refers to a cut substrate having a particular orientation with respect to a quartz crystal axis, and is widely used, not only in the QCM device, because AT cut has a temperature coefficient whose change becomes minimum at temperatures near ambient temperature and thereby has high temperature stability.
The AT cut quartz crystal resonator operates in so-called thickness-shear vibration mode in which the front surface and the back surface slide alternately when a voltage is applied between the excitation electrodes formed on the front and back of the substrate. The resonance frequency f0 thereof is inversely proportional to the quartz crystal board thickness of an area sandwiched between the front and back electrodes, and the following relationship generally holds.f0(MHz)=1670/quartz crystal board thickness (μm)
In addition, it is known that the relationship between the amount ΔM of adsorbed substance and the magnitude Δf of the frequency change of the QCM device using the AT cut quartz crystal resonator is expressed by the following Sauerbrey equation:
      Δ    ⁢                  ⁢    f    =            -                        2          ×                      f            0            2                                                ρ            ×            μ                                ×                  Δ        ⁢                                  ⁢        M            A      wherein f0 is the resonance frequency of the resonator, ρ is the density of quartz crystal, μ is the shear modulus of quartz crystal, and A is the effective oscillating area (which is nearly equal to the electrode area). As is clear from the equation above, by increasing the resonance frequency f0 of the quartz crystal resonator, it is possible to increase the sensitivity, that is, the magnitude Δf of the frequency change per amount ΔM of adsorbed substance.
Incidentally, an odor sensor using a QCM device has been disclosed in JP-A-63-222248 (Patent Document 1), for example. In the technique disclosed in Example 6 of Patent Document 1, the presence of an odorous substance β-ionone with which the air is saturated is detected as a change in the frequency (FIG. 9) by using an element (FIG. 6) provided with an AT cut quartz crystal resonator 1 having an electrode 2 on which a bilayer membrane film formed of dialkylammonium salt and polystyrene sulfonic acid is formed as an adsorption film.
However, in reality, it is impossible to make the adsorption film selectively adsorb only a target substance, and a plurality of substances are adsorbed onto the adsorption film. To solve this problem, in the technique disclosed in JP-A-1-244335 (Patent Document 2), for example, a so-called multiarray method is adopted by which the type of odor is identified by concurrently observing the changes in the frequencies of a plurality of quartz crystal resonators on which different types of adsorption films are formed and performing pattern analysis of the ratio between the changes in the frequencies of the resonators.
Here, to improve the accuracy of identification of odors, increasing the types of resonators used, the resonators provided with the adsorption films, is effective. However, 400,000 types of odorous substances to be measured are considered to exist, and it is impossible to identify such an enormous number of types of odorous substances by using several (several types of) resonators (QCM) as in the existing techniques described above.
When a large number of resonators (QCM) are used as described above, a drawing effect between the oscillation circuits connected to the resonators sometimes becomes a problem. The drawing effect here refers to a phenomenon in which, when the oscillation frequencies of two oscillation circuits are close to each other, something triggers a change by which one of the frequencies of the signals output from these oscillation circuits suddenly coincides with the other. When the drawing effect occurs, the frequency of the signal output from the oscillation circuit changes irrespective of the substance attached to the adsorption film.
In JP-A-10-142134 (Patent Document 3), to prevent the drawing effect, a plurality of sorted resonators having different resonance frequencies are used.
However, even when the resonators having different resonance frequencies are sorted and used as described in Patent Document 3, the characteristics such as load capacitance included in the oscillation circuit may vary. In this case, the frequency of the signal output from the oscillation circuit may not become an intended frequency, making it impossible to obtain a desired frequency difference between the oscillation circuits. This may also cause the drawing effect.
Furthermore, when a large number of resonators are used, the resonators have to have different resonance frequencies. As a result, it takes time to perform sorting.